1. Field of the Invention
The present invention relates to readout of detector signals from infrared imaging arrays and, in particular, to charge-coupled device utilization in imaging systems.
2. Description of Related Art
Decriptions of various infrared sensing devices, including but not limited to imaging devices formed on semiconducting substrates, can be found in The Infrared Handbook, by Wolfe et al., Office of Naval Research, Department of the Navy, 1978, particularly on pages 12-27 through 12-54. Typically, such an IR imaging device comprises an array of detectors whose outputs are converted into signals which are proportional to the amounts of IR radiation received during the integration time, then sent to an array of parallel charge-coupled device (CCD) serial registers. The CCD serial registers receive charge from photodetectors which are adjacent on the substrate. Often the parallel CCD registers are arranged in vertical columns, with their outputs connected to a single horizontal CCD register which acts as the multiplexer.
Once during each CCD clock cycle a small amount of charge, commonly referred to as a "fat zero charge packet," is injected into each column CCD register. This is done to increase charge transfer efficiency. In the ideal case the amount of charge contained in each fat zero chare packet is constant. The column CCD registers are typically connected to a common input gate through which the fat zero charge packet can be separately introduced into each of the column registers.
The signal generated by CCD imagers is affected by several sources of noise. One such source is fast interface state noise (also called "FIS noise" or "surface state noise"); it is described on pages 111 and 112 of the book Charge Transfer Devices, by Sequin et al., published by Academic Press, Inc., in 1975 in New York. Interface noise arises out of a phenomenon in which random trapping of CCD charge carriers takes place in crystalline surface states in the interface between the semiconductor substrate surface and the overlying insulating layer. The effect is that the amount of transfered charge fluctuates, and the fluctuations result in FIS noise.
Another source of noise is electrical injection noise, also described in the cited reference by Sequin, on page 116. Electrical injection noise is the result of jitter in the voltages applied to control the operation of the charge-coupled device. The capacity of an electrical potential well in a semiconductor substrate to hold charge varies with the fluctuations of the voltage applied to the gate electrode underneath which the potential well is formed. Electrical injection noise is generated by such fluctuations. Both interface state noise and electrical injection noise are important considerations if the CCD imager views a scene that has a low background radiation level.
In cases where the background radiation level is high, another significant noise source is background photon noise. This topic is treated on pages 11-18, 11-19, and 11-39 through 11-44 of the reference by Wolfe et al. cited above. Background photon noise is caused by fluctuations in the background radiation level, and is proportional to the square root of the number of photons or photon-generated carriers. If the detector gain is reduced to compensate for high levels of background radiation, the number of photon-generated carriers is reduced and the signal-to-noise ratio decreases.
Two critical requirements for space sensor applications are low-noise readout of the detector signals and the ability to perform low-power analog time-delay-and-integration (TDI) on the sensor chip. Low-noise readout circuits must maintain nearly constant detector bias for accurate signal conversion with sensitive narrow-bandgap detector semiconductors such as indium antimonide and mercury cadmium telluride. Focal plane detector arrays currently being designed involve very large numbers of densely packed IR detector elements. The high packing density requires low-power analog signal processing (such as charge-domain TDI) on the chip to reduce spacecraft power consumption for cooling.
Several techniques exist for encoding output from a semiconductor IR detector into charge packet form.
As shown in FIG. 1, the method of direct injection involves filling the potential well under the middle gate electrode designated .phi..sub.2 and then clocking the charge into the CCD channel. With direct injection the detector "sees" a high impedance, since the input MOSFET formed by the diffusion and the V.sub.BIAS gate is operating in subthreshold. The detector bias therefore varies with signal. Fluctuations of the 1/f type of the surface potential under the V.sub.BIAS gate modulates the detector bias, and for low-impedance detectors also modulates the charge passed to the .phi..sub.2 well. FIG. 2 shows another technique used for fat zero injection; this technique is subject to random variation in the spilling of charge as the .phi..sub.1 gate surface potential is raised to isolate the charge packet in the .phi..sub.2 well.
The method of buffered direct injection (also called the synthetic transimpedance amplifier method) is shown in FIG. 3. This method reduces both detector bias variation with signal current and the readout circuit input impedance by a factor of G, the gain of the inverting buffer. However, 1/f noise from the input MOSFET modulates the detector bias and hence the detector current form low-impedance detectors.
The direct injection method is improved on by the Tompsett sampler charge injection circuit, which is shown in FIG. 4. The potential well under the V.sub.IN gate is filled and then spilled by pulsing .phi..sub.DIFF and .phi..sub.GATE. The amount of charge left in the well depends on the difference in voltage between V.sub.IN and .phi..sub.GATE. The surface potentials under both V.sub.IN and .phi..sub.GATE gates give rise to 1/f noise that causes the size of the well to fluctuate. An improved version of the Tompsett sampler charge injection circuit is shown in FIG. 5, embodying a well gate with a much smaller area to reduce the effects of random fluctuations in the surface potential under the gate V.sub.IN.sbsb.1. A wide gate V.sub.IN.sbsb.2 is provided to allow full dynamic range for test purposes, as shown in FIG. 6, but would not be used for sensitive infrared detection applications.
None of the above methods of chare injection allows the processing of detector signals having a large dynamic range while operating with low 1/f noise and minimal detector bias variation, as well as also offering the possibility of being used for fat zero charge injection so that fluctuating surface potentials in the metal oxide semiconductor structure present less of a problem. There has long been a need for a method and apparatus for the conversion of a small current from a photodetector into a precisely proportional charge packet and its injection into a CCD channel. Such a method and apparatus would be a boon to the technology of reading out the signals from IR detector arrays, which is so important for critical remote sensing applications.